1. Field of the Invention
The present invention relates to a jointing structure for superconducting cable and a method for jointing the superconducting cable, and more particularly to an intermediate joint structure of a superconducting cable that prevents cracks by remarkably improving: physical bonding strength of a jointing interface, a rise in temperature of the refrigerant, and overloading of a cooling system due to such a temperature rise, as well as having excellent electrical characteristics, and a jointing method thereof.
2. Description of the Related Art
Generally, a power cable using a superconductor transmits a great amount of electric power and has very small power loss in comparison with a conventional power cable.
For the purpose of transmitting a large amount of electric power over a long distance by using the superconducting cable, an intermediate jointing member between superconducting cables is absolutely necessary. The intermediate jointing member includes a conductor-ointing section, an insulating section, and a refrigerant circulation structure for cooling the superconducting cable under a critical temperature.
A conventional technology for the conductor jointing section is disclosed in Japanese Patent Laid-open Publication No. HEI 4-98773, and will be briefly described with reference to FIG. 1 as follows.
The conventional conductor-jointing member is constructed as follows. First, inner formers 41 and 51 of a cable are cut to a length shorter than the superconductor tapes 42 and 52 surrounding the formers 41 and 51. A jointing member 4 with a laminated structure identical to that of the cable is inserted between the formers 41 and 51 of both cables. Finally, the formers 41 and 51 of the cables are jointed at both ends of the jointing member 4 by silver solders 8 and 9, so that the superconductor tapes 42 and 52 are jointed with each other by silver solder 7.
The jointing member 4 is formed by winding superconductor tape 62 around a metal pipe 61 with a laminated structure identical to the cables. The superconductor tape 62 is constructed by sequentially laminating a conductive metal layer 63, a superconductor layer 65, and a conductive metal layer 64 thereon.
Therefore, in the conductor-jointing section, electric current supplied to the superconductor layer 45 of one cable flows to the superconductor layer 55 of the other cable via the superconductor layer 65 of the jointing member 4.
However, in the conventional method for jointing cables, since the jointing member is jointed with the formers of the cables by silver solder, cracks may be generated at an interface between the jointing member and the cables, due to heat shrinkage during a cooling operation below a critical temperature. If a refrigerant infiltrates the fragile interface and a cracked region under high pressure (about 10 bar), the jointing section may fracture or its contact resistance may increase. Moreover, due to the temperature rise caused by Joule's heat, the cooling load of the cooling system increases, as well, a quench phenomenon of a superconductor may arise.
Moreover, since temperature excessively rises when both ends of the jointing member are jointed with a former by the silver solder, the superconductor tape may become deteriorated.